Influence of Straight Nozzles on Fluid Flow in Mold and Billet Quality
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ters are aware of the importance of the fluid flow effects on superficial and internal quality of steel, originating many publications particularly related with the influence of submerged entry nozzles on steel flow in slab molds.[1–10] Fewer publications related with fluid flow in billet molds and particle image velocimetry (PIV) measurements are available,[11–15] probably because the conventional straight nozzles employed in this field work in a much more confined space, inducing what would be called standard flows with small variations of fluid flow patterns. However, the study presented here made clear that the complex nature of turbulent flows prevails even under the conditions of very stable flows, such as those expected in billet molds. Specifically, the case treated in this work corresponds to a caster in Mexico, hereinafter called simply Company ‘‘A,’’ which operates a four-strand billet caster. Ladle capacity is 70 tons of steel that transfers the metal to a delta-shaped tundish with a capacity of 18 tons. Billet section is 160 9 160 mm and Figures 1(a) through (c) show the E. TORRES-ALONSO, S. GARCIA-HERNANDEZ, and A. NAJERA-BASTIDA, Graduate Students, and A. SANDOVALRAMOS, Researcher, are with Department of Metallurgy, Instituto Polite´cnico Nacional-ESIQIE, Mexico D.F., CP 07738, Mexico. R.D. MORALES, Professor, Department of Metallurgy, Instituto Politecnico Nacional-ESIQIE, is President, K&E Technologies, And. 8 No. 7 Col. El Risco-CTM, Me´xico D.F., CP 07079, Mexico. Contact e-mail: [email protected] Manuscript submitted November 20, 2007. Article published online November 15, 2008. 840—VOLUME 39B, DECEMBER 2008
mold geometry. Machine radius is 7418 mm and the mold corners have a radius of 4 mm. Production mix includes all sorts of low-alloy special steels including Cr, Cr-Ni, Cr-Ni-Mo, and Cr-V grades as well as medium carbon steels used for heat-treated automotive components. Steel refining in Company A is carried out through the EAF-LF-VD route. Average monthly production is 35,000 tons yielding a production of steel per year of 420,000 tons. Steel feeding into the molds is performed indistinctly through straight nozzles with the same internal diameter of 36 mm; the only difference among them is their outer diameters, which are 60 and 73 mm, as is seen in Figures 2(a) through (b). These nozzles are called S60 and S73, respectively. Minimum and maximum nozzle operating submergences of these nozzles are 90 and 135 mm measured from the meniscus level to the nozzle tip. Lubrication powder employed in the mold has the chemistry shown in Table I. The problem hindering the billet quality was the irregular but permanent entrainment of flux into the metal bulk, as was proved through chemical mappings obtained by a scanning electron microscope (SEM). Figures 3(a) through (f) show a secondary electron image of one of these macroinclusions and the corresponding chemical mappings where the presence of sodium is abundant, demonstrating the mold powder as the origin of such defects. Spent nozzles presented the usual corrosion belts
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